Lightning strikes governed by moving cloud layers

Zeus's weapon has been cracked. Researchers say they have finally worked out how lightning forms, how it escapes the storm cloud that fosters it - and whether it will form a "bolt from the blue".

The theory, which is backed by a computer model, is the first to explain all of the different types of lightning that exist - from regular cloud-to-ground lightning, to gigantic "jet lightning" that escapes up through the top of clouds, to "bolts from the blue", which can strike the ground under blue skies, several kilometres away from a thunderstorm.

Scientists knew that lightning forms between layers of positive and negative charges within a cloud. But this could not explain how lightning ever made it out of the cloud.

"According to those theories, all lightning should remain trapped between the charged layers, inside clouds," says Riousset. "We had suspicions about why it would go towards the ground, but we had no idea why it would go upwards."

Charged up

Riousset's model shows that when the layers of positive and negative charges inside a cloud are redistributed, this can create the conditions necessary for lightning to escape by amplifying the difference in potential between the layers.

The direction in which it escapes depends on how the charges were initially distributed inside the cloud, their magnitude, and the weird fact that charges move upwards more readily than they move towards the ground.

A lightning bolt can even be re-directed from its original path, as happens with bolts-from-the-blue. These massive upwards jets get re-routed by charges lining the sides of the cloud. The lightning ends up exiting the cloud sideways and can hit the ground more than 40 kilometres away from a storm.

Bolt down

Riousset says there are three ways in which the charges inside a storm cloud might be redistributed. Altitude is one. Charges are thought to dissipate at different rates depending on their altitude, meaning layers can build up inside the cloud.

In addition, intra-cloud lightning moves charges around inside the cloud, and cloud-to-ground lightning removes charges by channelling them down to the ground. Both of these disturb the charges further, creating the conditions for lightning to escape upwards.

Riousset says the computer model he and his colleagues have developed can be used to predict what type of lightning a storm cloud is likely to generate. This means they can say if the lightening will escape upwards, hit the ground beneath the cloud, or strike Earth miles away from the storm.

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.